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stats_stat_percentile

(PECL stats >= 1.0.0)

stats_stat_percentile — Returns the percentile value

说明

stats_stat_percentile ( array $arr , float $perc ) : float

Returns the perc-th percentile value of the array arr.

参数

arr: The input array
perc: The percentile

返回值

Returns the percentile values of the input array.

User Contributed Notes

aboulang2002 at yahoo dot com 28-Jan-2009 05:22


If you are looking to infer the percentile from a z-score, you can use this function.


It's far from precise but does the job on most circumstances.


The error function ( erf() )is based on the approximation on wikipedia: 


http://en.wikipedia.org/wiki/Error_function





<?php


function erf($x)


{


        $pi = 3.1415927;


        $a = (8*($pi - 3))/(3*$pi*(4 - $pi));


        $x2 = $x * $x;





        $ax2 = $a * $x2;


        $num = (4/$pi) + $ax2;


        $denom = 1 + $ax2;





        $inner = (-$x2)*$num/$denom;


        $erf2 = 1 - exp($inner);





        return sqrt($erf2);


}





function cdf($n)


{


        if($n < 0)


        {


                return (1 - erf($n / sqrt(2)))/2;


        }


        else


        {


                return (1 + erf($n / sqrt(2)))/2;


        }


}





// EXAMPLE


$sample = 90;


$avg = 75;


$stddev = 12;





$zscore = ($sample - $avg) / $stddev;


print 'Percentile: ' . cdf($zscore) * 100 . "\n";


?>





Where $n is the z-score





The function cdf() returns the approximed cumulative standard normal distribution ([0..1])











[EDIT BY danbrown AT php DOT net: Contains a bugfix provided by (Ed) on 24-FEB-2010 which fixes the definition of $a in erf() with the note that it is "out by a factor of -1" in the original code.]

yuvaraj dot v at gmail dot com 11-Dec-2007 04:37


I have looked at the code available in the Math package for percentile. I compared the result obtained with percentile from Excel and found that it doesnt match. So i wrote my own percentile function and verified the result with the Excel's percentile.





For those of you who need the percentile calculation of Excel in php..





<?php


function mypercentile($data,$percentile){


    if( 0 < $percentile && $percentile < 1 ) {


        $p = $percentile;


    }else if( 1 < $percentile && $percentile <= 100 ) {


        $p = $percentile * .01;


    }else {


        return "";


    }


    $count = count($data);


    $allindex = ($count-1)*$p;


    $intvalindex = intval($allindex);


    $floatval = $allindex - $intvalindex;


    sort($data);


    if(!is_float($floatval)){


        $result = $data[$intvalindex];


    }else {


        if($count > $intvalindex+1)


            $result = $floatval*($data[$intvalindex+1] - $data[$intvalindex]) + $data[$intvalindex];


        else


            $result = $data[$intvalindex];


    }


    return $result;


}


?>





The above code may not be elegant.. but it solves my problem..





yuvaraj

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stats_stat_percentile

说明

参数

返回值

User Contributed Notes

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